Soil fertility management is a major concern in organic farming. According to the EEC Regulation 2092/9, soil fertility must be maintained and/or increased by legumes cultivation. Native Mediterranean legumes ecotypes are more persistent than commercial varieties but their use in catch crop sequences is still limited. The N2-fixing legumes crops as a local resource can be considered as an alternative to improve nitrogen nutrition and to increase crop yields. This study aimed to investigate the agronomic performance of seven annual self-reseeding legumes and the short–term effect of nitrogen supply on the subsequent crops. Results are presented from field experiment on sandy-clay soil, pH 8.1 and 1.6% of organic matter, in the south of Italy, where the self-reseeding legumes were evaluated during two cropping cycles for their adaptability and integration into Mediterranean cropping systems. Biological nitrogen fixation (BNF) was also performed using 15N isotopic dilution method.
For the first cropping cycle, Trifolium spp. performed better than Medicago spp. and of the seven tested species, five were more suitable to the site conditions. These species could be used for managing soil fertility and enhancing biodiversity in orchards. Trifolium angustifolium and Medicago polymorpha were the most performing species. They fixed 132 and 90 kg N ha-1 of BNF, produced 1976 and 731 kg ha-1 of seeds and 8.7 and 5.5 t ha-1 of dry biomass (DM) respectively. In contrast, Medicago radiata and M. rigidula were the least performing species.
For the second cropping cycle, their self-establishment capacity and their effect as green manure on two subsequent crops of zucchini and lettuce were investigated. Results showed again that Trifolium spp. performed better than Medicago. Except for M. radiata and M. rigidula, regenerated species appear to sustain optimum level of soil-available nitrogen and induced positive effects on both zucchini and lettuce growth parameters. Zucchini and lettuce marketable yield significantly increased influenced by the preceding legumes, on average 53% and 24% respectively over the control. T. angustifolium was again the best performing and promising species producing the highest DM (7.74 t ha-1) and fixing nitrogen (146.7 kg N ha-1) symbiotically. It induced the best effect on all zucchini and lettuce parameters including crop yield (42.66 and 48 t ha-1 respectively). In contrast, M. polymorpha was less performing (0.3 t ha-1 of DM and 11.5 kg ha-1 of BNF). Given the overall performance of all species, it was determined that T. angustifolium had the greatest potential for further development in this environment....more